Waveguides are used to guide electromagnetic radiation from one component of a device to another or to guide radiation within a component. One example of device which utilises a waveguide is a spectrometer.
Spectrometers are used in many applications for measuring properties of light across a range of wavelengths. For example, a spectrometer can be used for compositional analysis, by obtaining absorption or emission spectra for an object of interest. The presence and location of peaks within the spectra can indicate the presence of particular elements or compounds. Spectrometers are commonly used for analysis at optical wavelengths, but can also be used at other wavelengths such as microwave and radio wavelengths.
Spectrometers are typically relatively complex and expensive devices that require the alignment of a number of moving parts to be controlled with high precision. For example, a typical spectrometer may focus light onto a diffraction grating to split an incident beam into separate wavelengths, and the diffraction grating may be rotated to a specific angle to direct light of a particular wavelength towards a detector. In recent years chip-based spectrometers have been developed which can be highly miniaturised, have no moving parts, and can be manufactured using well-established lithography techniques. An example of such a spectrometer-on-a-chip is shown in FIG. 1.
The chip spectrometer 100 comprises a substrate 110, onto which are patterned a waveguide 120 and a plurality of disk resonators 130 coupled to the waveguide. Light enters the waveguide at a first end 120a and is guided towards a second end 120b. The resonators are arranged such that portions of the light in the waveguide are coupled into the disk resonators 130. Each resonator 130 is arranged to support a resonant mode at a particular wavelength such that only light of that wavelength is coupled into the resonator 130. On top of each disk resonator 130 is an electrode 140 for detecting current that is proportional to the amount of light present in that resonator. The current detected in each resonator therefore indicates the amount of light at that wavelength that was present in the input beam of light. Each electrode 140 is further connected to a signal bond pad 150 for connecting the spectrometer 100 to an external device for measuring the current. A portion of the light is not coupled into any of the resonators and reaches the second end 120b of the waveguide. Back-reflections from the end of the waveguide may give rise to interference within the spectrometer chip which degrades the performance of the spectrometer. A low reflective coating 160 is therefore evaporated or sputtered onto the second end 120b of the waveguide to stop back-reflections from light reaching the end of the waveguide. However, the application of the low reflective coating requires an additional processing step in the manufacture of the spectrometer.
The invention aims to improve on the prior art.